Impedance-increasing method and apparatus as part of a hvdc circuit breaker

ABSTRACT

The high voltage DC circuit breaker must generate a variable impedance which increases gradually from negligibly small, when the breaker is closed, to infinitely large, after it has been opened. This invention describes a novel impedance-increasing method comprising serially connected resistance and switch means and a capacitor which shunts both. The switch means, which must be capable of opening and closing a high voltage, is programmed to rapidly switch on and off with increasing offtime so that the time averaged impedance of the DC circuit is increased to an offswitched condition.

United States Patent Knauer [151 3,657,607 Apr. 18, 1972 [54] IMPEDANCE-INCREASING METHOD AND APPARATUS AS PART OF A HVDC 3,198,986 8/1965 Luehringetal. ..317/l1E 3,534,226 10/1970 Lian ..3l7/I1C CIRCUIT BREAKER Primary Exammer.lames D. Trammell [72] Inventor: Wolfgang Knauer, Malibu, Calif. A!t0rneyW. H. MacAllister, Jr. and Allen A. Dicke. Jr.

[73] Assigncc: Hughes Aircraft Company, Culver City. [57] ABSTRACT Calif. The high voltage DC circuit breaker must generate a variable [22] Wed: 9,1971 impedance which increases gradually from negligibly small, [21 APPL 122,396 when the breaker is closed, to infinitely large, after it has been opened. This invention describes a novel impedance-increasing method comprising serially connected resistance and [52] U.S. Cl. ..317/11 C, 307/136 switch means and a capacitor which shunts both. The switch [51] Int. Cl..- ..ll02h 7/22 means, which must be capable of opening and closing a high [58] Field of Search ..317/11 A, 11 C, 11 E; 307/133, voltage, is programmed to rapidly switch on and off with in- 307/136; 200/ 144 AP creasing offtime so that the time averaged impedance of the DC circuit is increased to an offswitched condition. 6 R f d [5 1 e erences cm 10 Claims, 9 Drawing Figures UNITED STATES PATENTS 2,849,659 8/1958 Kesselring...- ..317/11 A 22 a 8 M W W Rectifier I Transformerl-{Ienerut r l [i I I8 i l6 l4 12, I I0 36 26 30 I IT ea-""1 28 I i l l l PATENTEUAPR 18 [972 3,657, 607 SHEET 2 OF 4 PATENTEBAPR 18 I972 SHEET 30F 4 IMPEDANCE-INCREASING METHOD AND APPARATUS AS PART OF A I'IVDC CIRCUIT BREAKER BACKGROUND This invention is directed to an impedance-increasing methodand apparatus of a high voltage DC circuit breaker which operates in conjunction with an in-line switch. To initiate offswitching, the in-line device which carries the normal current flow, is opened and the current flow is transferred to the impedance-increasing section of the circuit breaker.

The prior art includes Kenneth T. Lian US. Pat. No. 3,534,226, wherein it was first recognized that a high voltage DC circuit breaker may comprise an in-line device which carries normal "circuit current when it is closed and thus presents low impedance to the current flow, and an impedance-increasing section to which the DC current is transferred from the inline device as the in-line device becomes nonconductive. The impedance-increasing section can comprise several different characterizations of impedance-increasing circuitry. The above-mentioned Lian US. Pat. No. 3,534,266 teaches the increase of impedance by employing a plurality of resistors and a switch in connection with each of the resistors. The net circuit impedance 'is increased as the switches are successively opened. Thus, circuit impedance has a progressive net increase to finally permit cutoff of the current flow.

Both the Lian invention and this invention are capable of being used as AC offswitching circuit breakers. However, since alternating current has a natural current zero, the offswitching thereof is a less complex situation than found in DC circuitry. It would be useful to employ this invention only where it is desired that offswitching occur before the next current zero is attained.

Contrasted to the Lian patent, which utilizes a number of resistance and switch means in a timed sequence, this invention is directed to the repetitive resistance and employment of switch means which cyclically open and close with increasing nonconductive time so that the time averaged impedance progressively increases, together with a capacitor which receives the circuit current when the switch is open.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to an impedance-increasing method and apparatus as part of a high voltage DC circuit breaker. The impedance-increasing apparatus comprises switching means having a resistance device serially connected therewith and a shunt capacitor. The switching device is driven on and off with an increasing proportion of offswitched time so that the short time averaged circuit impedance is progressively increased to a point where full offswitching can be accomplished. During off-periods, the current passes into the capacitor, thus preventing excessive voltage buildup. The method comprises the utilization thereof.

Accordingly, it is an object of this invention to provide a DC circuit breaker for the ol'fswitching of direct current circuits. It is a further object to provide a circuit breaker which employs a combination of in-line device and impedance-increasing device. It is still another object to provide an impedanceincreasing device wherein a switch, resistance, and capacitor are connected so that cyclic operation of the switch with varying on/off switch time ratios provides an increasing line impedance. It is still another object to progressively increase the circuit impedance until the circuit current is decreased to zero and the circuit is fully opened.

Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims, and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of a high voltage DC circuit, showing one embodiment of the impedance-increasing method and apparatus as part of a high voltage DC circuit breaker therein.

FIG. 2 is a graph showing line voltage to ground versus time during a circuit breaker opening cycle, when the circuit breaker resistance is linear.

FIG. 3 is a graph showing line current versus time for the opening cycle of FIG. 2.

FIG. 4 is similar to FIG. 2, but showing the voltage curve with the employment of a non-linear resistor in the circuit of this invention.

FIG. 5 is a curve showing the line current versus time for the curve of FIG. 4.

FIG. 6 is a schematic diagram of a further circuit breaker, in accordance with this invention.

FIG. 7 is a schematic diagram of a further circuit breaker of this invention.

FIG. 8 is a schematic diagram of a further circuit breaker, in accordance with this invention.

FIG. 9 is a schematic diagram of a further circuit breaker, in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the DC power which is to be ofi switched by the circuit breaker of this invention having an impedance-increasing means therein is conventionally derived at power source 10 which delivers power to AC generator 12. Generator l2 delivers its output to transformer 14 by which the voltage is raised to suitable transmission line voltage. From the transformer, the power is rectified by rectifier 16. Rectifier 16 has positive and negative output lines 18 and 20 respectively. Inductance 22, connected in the lines and capacitance 24, connected between the lines, serve as conventional DC filtering and smoothing equipment. They are preferably connected at the output of the rectifier, as shown. In certain circumstances, the reactance of the transmission system may be sufficient to provide adequate smoothing for economic power transmission.

Circuit breaker 26 is serially connected in line 18 between the rectifier 16 and transmission system 30, while an identical circuit breaker 28 is connected in line 20 therebetween. Bipolar circuit breakers are thus provided, because of the high voltages in the exemplary embodiment of employment of the circuit breaker. In lower voltage systems, only one circuit breaker might be necessary.

In high voltage DC systems, it is customary to have a line potential such that one line is above ground potential, while the other is below. This equalizes the amount of transmission line insulation between the two lines and ground. Either one of the lines, through the transmission system or at the load, may fault to each other, as by exemplary fault switch 32, or can fault to ground. Thus, independent line protection is necessary, as by breakers 26 and 28.

Circuit breaker 26 comprises line switch means 36 and impedance-increasing means 38. Line switch means 36 comprises switch 40 which is serially connected in power line 18 between rectifier 16 and the load 34. Line switch means 36 is of such nature that, as it opens, sufficient voltage drop is generated thereacross to transfer the current from the low impedance path through normally closed switch 40 into the impedance-increasing means 38. Different devices suitable for employment as switch 40 include conventional circuit breakers, of which Westinghouse Circuit Breaker Type 1150 SF 10000 is an example. This circuit breaker is rated for KVRMS at 2,000 amperes continuous current. Two of such breakers in series would be suitable as a switch in the 200 KV example given below. A vacuum relay, such as Allis Chalmers part No. VSC-l5, can serve as switch 40, provided that it is equipped with circuitry which induces an artificial current zero so that the metal are therein is permitted to deionize. A particular current transfer circuit of this nature is disclosed in Michael A. Lutz and Willis F. Long patent application, entitled Current Transfer Circuit As Part of High Voltage DC Circuit Breaker," filed concurrently herewith, Ser. No. 122,395. The entire disclosure of that application is incorporated herein by this reference.

In any event, the line switch means 36 is conductive during normal conduction so as to provide for a very low impedance normal conduction path. Upon fault detection, or for voluntary circuit opening, switch 40 is opened and the current flowing therethrough is transferred to the impedance-increasing means 38, which is thereafter actuated to increase the circuit impedance between the source and the load to ultimately cut off the circuit continuity.

FIG. 1 illustrates a simple impedance-increasing means 38 to show the most simple form of the method and apparatus of this invention. Cyclic switch 42 is serially connected to energy-absorbing resistance 44. It is understood that, as cyclic switch 42 is opened and closed, with an increasing ratio of open time to closed time, the time averaged impedance of these circuits increases. In order to receive the current during offswitch time and limit surges, smoothing capacitor 46 is paralleled around cyclic switch 42 and its energy-absorbing resistor 44.

' than 1, and can tolerate a maximum short circuit current 1 If a sudden short circuit develops, the current varies with time Thus, offswitching must be initiated not later than at time t The maximum resistance which can be inserted at time t is given by max max

max

If a resistance larger than R, is used, the voltage will rise above the insulation level of the line; with a smaller R, the voltage will remain below U In the latter case, the short circuit current decreases at a rate slower than with R,,, as follows: from I Accordingly, the shortest offswitching time t,,,,,, is obtained with U U,,,,,,. This time t,,,,,, is given by ITIZIX L r) f 1 It is evident that, during offswitching, the load impedance must increase with time in order that the voltage can be kept constant at U,,,,,,. With the help of several of the above relamin Y tions, the resistance can be computed to vary as ris, was

Thus, for t= t,,,,,,, the resistance R goes to infinity, as must be the case for a totally disrupted circuit. It is concluded that, for circuit breakers which offswitch in minimum time and which therefore pass the minimum energy, the design objective should be to reach a temporal dependence of R equivalent to that given by the last equation.

Control means for the cyclic switch 42 is provided to switch it substantially in the time dependance given in the above equation. FIGS. 2 and 3 show a representative switching sequence for the circuit shown in FIG. 1. The numerical values chosen for this example are: line inductance L 2 Henry, operating voltage U, 200 KV, operating current 1,,

1,000 amperes, over-voltage factor F 1.7, maximum short circuit current 1,, 2,000 amperes. The parameters of the switch circuit are: the impedance of resistance 44 R, 170 ohms. Similarly, capacitance C of the capacitor 46 equals 2 microfarads. The curves of FIG. 2 represent the voltage in line 18 at the output of rectifier 16 with respect to ground. FIG. 3 represents the current in line 18 at the output of the rectifier. At zero time in these curves, the voltage and current are normal at full load. At about 1 millisecond, the fault occurs and, between 1 millisecond and 10 milliseconds, the current is transferred from the line switch means 36 into the impedanceincreasing means 38, and line switch 40 is opened and deionized. The switching sequence of these figures was arbitrarily timed to let the voltage drop to l /e of the over-voltage U minus U during each cycle. The first drop in voltage to about 21 milliseconds has a shallower slope than subsequent switching cycles because the line current still has a large value and thereby keeps the capacitor 46 charged. From about 21 milliseconds, for the next 29 milliseconds, cyclic switch 42 is cycled so that the voltage varies between U m I and U At the beginning of this period, the cycle period is about 0.5 milliseconds and at the end, is about 2 milliseconds. There are about cycles of cyclic switch 42 in this 29 millisecond cycling period. Finally, at 50 milliseconds, cyclic switch 42 can be opened without the surge voltage exceeding U so that the circuit breaker is finally opened. FIG. 3 illustrates the progressive decrease in current with the progressively increasing time averaged impedance provided by cyclic switch 42 and its energy-absorbing resistor 44.

It has been shown above that, if the load impedance increases with time, so that the voltage is kept constant at U the cyclic offswitching time is decreased. Thus, when a current-dependent resistor, for example a thyrite" is employed as the resistor 44 and the value thereof is about R CI, fewer switching cycles and a shorter switching time is obtained. This is illustrated in FIGS. 4 and 5, which represent the offswitching of the circuit of FIG. 1 when resistance 44 is a current-dependent resistor of the value noted above. In FIGS. 4 and 5, it takes 35 milliseconds from the initial closure of cyclic switch 42 for the current and voltage to drop so that U, is reached. Thereafter, only about 15 cycles of the cyclic switch 42 are required to bring the current down so that cyclic switch 42 can be permanently opened without circuit overvoltage.

The circuit breaker 26 of FIG. I is a fairly simplified device which illustrates the theory of operation. Going now to various embodiments of circuit breakers which are operatedin accordance with these teachings, FIG. 6 illustrates circuit breaker 48 which comprises line switch means 50 and impedance-increasing means 52. The line switch means 50 is identical to line switch means 36 and the impedance-increasing means is similar in that it comprises a capacitor 54 across the circuit to be opened, which is paralleled by switch 56, which has resistor 58 in series therewith. The resistor 58 corresponds to resistor 44, and either may be linear or nonlinear, as previously indicated. The preferred switch in this embodiment is a crossed field tube. The ofiswitching of the crossed field tube 56 against rising high voltages requires a parallel capacitor 60, which limits the rate of voltage rise to about 2 kilovolts per microsecond. For interruption of 2,000 amperes, suitable numerical values for the circuit of FIG. 6 are as follows. Capacitor 54 is 2 microfarads, capacitor 60 is l microfarad, and resistor 58 is ohms. The crossed field tube 56 is basically an offswitch and is disclosed in more detail in the above-mentioned Lian US. Pat. No. 3,534,226. It can be turned on with the proper magnetic field and a high voltage applied together with a suitable triggering method, such as the use of a plasma gun into the inter-electrode space. Details of this onswitching are disclosed in patent application Ser. No. 122,397, filed by Gunter A. G. Hofmann concurrently herewith, and entitled Method and Apparatus for Ignition of Crossed Field Switching Device for Use in a HVDC Circuit Breaker, the entire disclosure of which is incorporated herein by this reference. The crossed field tube described above has among its antecedents those described in Penning U.S. Pat. No. 2,182,736; Boucher U.S. Pat. Nos. 3,215,893 and 3,215,939; and Wasa U.S. Pat. No. 3,405,300.

Another embodiment of the circuit breaker, in accordance with this invention, is illustrated in FIG. 7, wherein the circuit breaker is identified at 62. Circuit breaker 62 comprises line switch means 64, which is identical to line switch means 36, and impedance-increasing means 66.

Impedance-increasing means 66 has capacitor 68 paralleled across the line switch, to control the rate of voltage rise, as described above. Also connected between the lines, or in the circuit of the circuit breaker, is a series combination of resistor 70, triggered vacuum gap 72, and resistor 74. Paralleled around triggered vacuum gap 72 is a series combination of resistance 76 and capacitor 78. Triggered vacuum gap 80 is connected to the'breaker input line and to the capacitor 78. Capacitor 82 is connected between the breaker input line and triggered gap 72.

The triggered vacuum gaps 72 and 80 are of the nature described in Lafferty U.S. Pat. No. 3,290,542. Such a triggered vacuum gap is basically an onswitch. In conduction, they operate in a metal are mode with a fairly low (50 volt) voltage drop. In order to turn them off, one must force the current to zero. One of the ways in which forced commutation can be accomplished is by the circuit shown in FIG. 7, in association with the triggered vacuum gaps. Triggered vacuum gap 72 is the main switch which must be periodically opened and closed. Assuming that triggered vacuum gap 72 is conductive, then the current passing through resistor 70 charges capacitor 82. Now, if triggered vacuum gap 80 is made conductive, a negative voltage pulse is transmitted through capacitor 78 to the other pole of triggered gap 72. This forces the current to zero and triggered gap 72 deionizes and becomes nonconductive. Triggered tap 80 continues to conduct, since the I charging current of capacitor 78 passes through it. If triggered gap 72 is fired again, the plus pole of triggered gap 80 is pulsed negatively via the charged capacitor 78. Thus, triggered gap 80 extinguishes and the next cycle can begin. It must be noted that only triggered gap 72 must withstand the maximum circuit voltage. Triggered gap 80 can have a relatively low voltage rating. Thus, impedance-increasing means 66 can be cycled to obtain a progressively increasing time average impedance into the circuit breaker circuit.

Suitable component values for the circuit of FIG. 7 are as follows: capacitor 68, 2 microfarads; capacitor '78, l microfarad; and capacitor 82 is microfarads and can be of low voltage. Furthermore, resistance 74 is 170 ohms, resistance 70 is 10 ohms, and resistance 76 is 10 ohms.

FIG. 8 illustrates the circuit breaker 84 which comprises line switch means 86 and impedance-increasing means 88. The line switch means 86 is identical to line switch means 36. The impedance-increasing means is arranged to cyclically insert resistance into the circuit breaker circuit so that the time averaged impedance progressively increases to previous current flow. The impedance-increasing means 88 is another circuit with a triggered vacuum gap. Here, triggered gaps 90 and 92, identical to triggered gaps 72 and 80, are connected through resistance 94 to one line and are respectively connected through resistances 96 and 98 to the other line.

Resistance 100 is connected in parallel to spark gap 102 to one line of the circuit breaker and, on the other end, they are both connected through capacitor 104 to the bottom of triggered gap 92. Additionally, the bottoms of these two triggered vacuum gaps 90 and 92 are connected together through capacitor 106. Again, capacitor 108 is connected between the lines to control the rate of voltage rise.

In FIG. 8, the two triggered vacuum gaps 90 and 92 alternately carry the current. Assuming that triggered vacuum gap 90 is initially conductive and triggered vacuum gap 92 is nonconductive, then capacitor 106 charges. Now, if triggered vacuum gap 92 is made conductive, capacitor 106 pulses triggered vacuum gap 90 to force commutate zero current therein. This causes deionization and nonconductiori of triggered vacuum gap 90. Now, capacitor 106 charges in the opposite direction. Thus, by alternate triggering, the other triggered vacuum gap is force commutated. When line current has sufiiciently decreased, triggered vacuum gap 92 can be force commutated to become nonconductive (while triggered vacuum gap 90 is already nonconductive) by the firing of the triggered spark gap 102.

In this circuit, both triggered vacuum gap devices 90 and 92 must have full voltage ratings, but only triggered vacuum gap 90 must be able to handle full current. Triggered vacuum gap 92 can be rated for currents about 1 order of magnitude smaller. Triggered spark gap 102 can be designed for comparatively small voltages.

Suitable values for the components shown in FIG. 8 are as follows. Capacitor 108 is 2 microfarads, capacitor 106 is 1 microfarad, capacitor 104 is 1 microfarad, capacitor 105 is 10 microfarads (at low voltage). Resistance 96 is 170 ohms, resistance 98 is 2,000 ohms, and resistances 94 and 100 are each 10 ohms.

FIG. 9 illustrates circuit breaker 110 which has a line switch means 112 and an impedance-increasing means 114. The line switch means 112 is identical to the line switch means 36. The impedance-increasing means 114 combines a triggered vacuum gap and a crossed field tube. Capacitor 116 is connected across the lines of the circuit breaker to control rate of voltage rise. Crossed field tube 118 and energy-absorbing resistor 120 are serially connected and they are connected between the lines of the circuit breaker. A crossed field tube 118 with ordinary ignition characteristics, without a plasma trigger, can be employed. Paralleled around the crossed field tube is capacitor 122. Paralleled around capacitor 122 is a series combination of inductance 124, triggered vacuum gap 126, and resistance 128.

The crossed field tube 118, without plasma trigger, is of such nature that it becomes conductive only if a relatively low voltage is applied to its electrodes, in the presence of the necessary magnetic field. Assuming that the crossed field tube 118 is initially noncondutive, then capacitor 122 is charged. Now, the triggered vacuum gap 126 is fired and a damped oscillation is initiated in the resonant circuit constituted by capacitor 122, inductance 124, and resistance 128. At the first current zero, the triggered vacuum gap 126 extinguishes. At this time, capacitor 122 is charged in the reverse direction. However, current passing through resistor tends to restore the original charge. As the capacitor voltage passes through zero, the crossed field tube ignites, when the magnetic field is appropriately on. After a period of conduction, the crossed field tube 118 can be ofiswitched and a new cycle begins.

In FIG. 9, suitable values of the components are: capacitor 116, 2 microfarads; capacitor 122, l microfarad; resistance 120, ohms; and resistance 128, 10 ohms. Additionally, inductance 124 is 10" Henrys.

This invention having been described in its preferred embodiment, it is clear that is is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is:

1. A high voltage DC circuit breaker, said circuit breaker comprising a line switch and an impedance-increasing means connected in parallel to said line switch so that, when said line switch is opened, current is transferred to said impedance-increasing means, the improvement comprising:

said impedance-increasing means comprising a cyclically operable switch and an energy-absorbing resistor in series with said cyclically operable switch and means to operate said cyclically operable switch, a rate of voltage rise limiting capacitor connected in parallel to said series combination of said cyclically operable switch and said energy-absorbing resistor, so that the time averaged impedance of said impedance-increasing means increases as said cyclically operable switch is operated to have an increasing ratio of offswitch time.

2. The circuit breaker of claim 1 wherein said cyclically operable switch is a crossed field switch device.

3. The circuit breaker of claim 2 wherein a rate of voltage rise limiting capacitor is connected in parallel to said crossed field switching device.

4. The circuit breaker of claim '3 wherein a triggered gap switching device has reactance means in series therewith, said triggered gap switching device and said reactance means being connected in parallel to said capacitor in parallel to said crossed field switching device so that onswitching of said triggered gap causes resonant pulsing of said crossed field switching device.

5. The circuit breaker of claim 1 wherein said switch means comprises a triggered vacuum gap switch device having said energy-absorbing resistance in series therewith.

6. The circuit breaker of claim 5 wherein there are first and second triggered vacuum gap switch devices connected in parallel to each other through reactance means so that said second triggered vacuum gap switch device induces a current zero in said first triggered vacuum gap switch device.

7. The circuit breaker of claim 6 wherein said reactance means comprises capacitive means connected between said first and second triggered vacuum gap switch devices so that said capacitive means is charged by voltage drop in said circuit breaker and said charge on said capacitive means is discharged through one of said triggered vacuum gap switch devices to impress acurrent zero on the other of said triggered vacuum gap switch devices. a

8. The process for increasing the impedance in a high voltage DC circuit breaker comprising the steps of:

cyclically opening and closing a switch having a resistance in series therewith, through which switch and resistance pass the current in the circuit breaker to be interrupted,

with increasing ratio of switch open time to switch closed 

1. A high voltage DC circuit breaker, said circuit breaker comprising a line switch and an impedance-increasing means connected in parallel to said line switch so that, when said line switch is opened, current is transferred to said impedanceincreasing means, the improvement comprising: said impedance-increasing means comprising a cyclically operable switch and an energy-absorbing resistor in series with said cyclically operable switch and means to operate said cyclically operable switch, a rate of voltage rise limiting capacitor connected in parallel to said series combination of said cyclically operable switch and said energy-absorbing resistor, so that the time averaged impedance of said impedanceincreasing means increases as said cyclically operable switch is operated to have an increasing ratio of offswitch time.
 2. The circuit breaker of claim 1 wherein said cyclically operable switch is a crossed field switch device.
 3. The circuit breaker of claim 2 wherein a rate of voltage rise limiting capacitor is connected in parallel to said crossed field switching device.
 4. The circuit breaker of claim 3 wherein a triggered gap switching device has reactance means in series therewith, said triggered gap switching device and said reactance means being connected in parallel to said capacitor in parallel to said crossed field switching device so that onswitching of said triggered gap causes resonant pulsing of said crossed field switching device.
 5. The circuit breaker of claim 1 wherein said switch means comprises a triggered vacuum gap switch device having said energy-absorbing resistance in series therewith.
 6. The circuit breaker of claim 5 wherein there are first and second triggered vacuum gap switch devices connected in parallel to each other through reactance means so that said second triggered vacuum gap switch device induces a current zero in said first triggered vacuum gap switch device.
 7. The circuit breaker of claim 6 wherein said reactance means comprises capacitive means connected between said first and second triggered vacuum gap switch devices so that said capacitive means is charged by voltage drop in said circuit breaker and said charge on said capacitive means is discharged through one of said triggered vacuum gap switch devices to impress a current zero on the other of said triggered vacuum gap switch devices.
 8. The process for increasing the impedance in a high voltage DC circuit breaker comprising the steps of: cyclically opening and closing a switch having a resistance in series therewith, through which switch and resistance pass the current in the circuit breaker to be interrupted, with increasing ratio of switch open time to switch closed time; and reactiVely damping current flow through the switch and the resistance to limit rate of voltage rise upon opening of the switch.
 9. The process of claim 8 wherein said reactively limiting step comprises the step of capacitively limiting rate of voltage rise.
 10. The process of claim 9 wherein closing is accomplished by the step of causing an electric discharge between spaced electrodes and switch opening is accomplished by the step of terminating the electric discharge between the spaced electrodes. 